In general, printing data whose printing is designated by an application program (CAD, word processing, DTP or the like) operating in a general-purpose data processing apparatus, e.g., a workstation (WS), a personal computer (PC) or the like, is expressed broadly by three object types: texts, halftone images such as photograph images, and graphics.
Texts include alphanumeric characters expressed by 1-byte codes, and characters like kanji (Chinese ideograms) expressed by 2-byte codes. Halftone images are constructed with color data arranged in two dimensions, which is different for each pixel. To increase transfer efficiency, halftone images are sometimes compressed. Graphics are expressed by lines, polygonal outlines, and filling areas.
Printing apparatuses adopting an electrophotography process have the following problem, which imposes a challenge in printing a clear thin line.
To print R, G, and B data designated by a PC, the data is converted to respective color data of cyan (C), magenta (M), yellow (Y) and black (K) in electrophotography method, and printing is performed by using toner that corresponds to the respective values. In the electrophotography method, for instance, in a case of printing a full secondary-color image with R=255, G=0 and B=0, printing is generally performed with Y+M (the total of 200% of Y and M, and 0% of C and K). However, in the process of image development and transfer, a phenomenon called toner spattering occurs, resulting in a blurred outline thicker than an actual line.
To solve this problem, for instance, in a case of printing an image with pure red color after RGB data is converted to YMCK data, it is a general procedure for a printer controller to perform printing with 90% Y and 90% M (Y, M=255×0.9≈230), i.e., the total of 180% data.
Even though the total value of C, M, Y and K is 180% toner adhesion amount, in a case of printing a large area, dot omission does not cause a big problem. However, in a case of printing a thin line (e.g., having a thickness of one or two dots), if the digital data is processed to 180% as described above, the thin line may be printed as a cracked line instead of a solid line after halftoning (dither, error diffusion and the like) is performed. In other words, data expressed as a solid line on a monitor will likely be printed as a cracked line, causing a problem.
A simple solution for solving this problem is to increase the thickness of the thin line to be thicker than a designated size, or to darken the color of the line to be darker than a designated color, thereby preventing a crack in the line. However, this is not a perfect solution of the problem because it is not exactly what the user has intended.